Conventionally, a valve spring of an internal combustion engine is attached as shown in FIG. 13, for example. FIG. 13 is a schematic sectional view showing a valve gear. In the valve gear of FIG. 13, a valve 101 is an intake valve or a discharge valve installed in a cylinder head 103 such that the valve 101 reciprocates through the cylinder head 103. A first end of the valve 101 is a spring retainer 105 serving as a movable part. The cylinder head 103 has a mounting seat 107 serving as a stationary part. Between the spring retainer 105 and the mounting seat 107, a valve spring 109 is arranged.
The first end of the valve 101 is in contact with a first end of a rocker arm 111. A second end of the rocker arm 111 is in contact with a cam shaft 113.
In response to rotation of the cam shaft 113, the rocker arm 111 rocks to make the valve 101 open and close an intake or discharge port.
At this time, the valve spring 109 applies spring force to the retainer 105 with respect to the seat 107 so that the first end of the valve 101 may surely follow the rocker arm 111.
FIG. 14 is a side view showing the valve spring 109, FIG. 15 is a plan view showing the valve spring 109, and FIG. 16 is a sectional view showing a relationship between a coil end and a first turn of the valve spring 109.
Recent valve springs are required to have high stress and high durability. To satisfy the requirements, the valve spring 109 has a vertical gap “t” between a coil end 115 and a first turn 119 of an element wire 117. With the gap t, the coil end 115 and first turn 119 of the element wire 117 are entirely processable by shot peening, to equally improve the residual stress of the first turn 119, prevent a fretting breakage of the first turn 119, and improve the durability of the valve spring 109.
The coil end 115 and first turn 119 come in contact with each other as shown with a chain double-dashed line in FIG. 16. At this time, load in an axial direction of the valve spring 109 directly acts as bearing force between the coil end 115 and the first turn 119. This increases Hertzian stress on the first turn 119 and deteriorates the durability of the valve spring 109.
The gap t is formed by deforming the coil end 115 in an axial direction of the valve spring 109 relative to the first turn 119. When the valve 101 is operated at high speed, the coil end 115 and first turn 119 rapidly repeat engagement and disengagement in the distance of t, to apply excessive bending stress to the coil end 115. This may break the coil end 115, or may cause a tip breakage of the coil end 115.    Patent Literature 1: Japanese Utility Model Patent Publication No. 2545359